To change the background, you just have to edit the screenshot URL for the Intro page section which starts like this –
[segment ... background_color='#374a4e' background_image="https://www.livemeshthemes.com/appdev/wp-content/uploads/2013/07/background1.jpg" parallax_background="true" background_speed="0.3"]
To specify the screenshots of your own for the iPhone slider, pls change the attributes of [iphone_slider] shortcode. You can use images of size 323px width and 553px height or something that scales in that aspect ratio.
More details in the documentation (latest version) for both of these shortcodes.